1. Field of the Invention
The present invention relates to an apparatus for measuring the luminescence lifetime of a sample on the basis of the measurement of the transient waveshape of light radiated from the sample excited by a pulse light, in particular the present invention relates to an apparatus for measuring the luminescence lifetime of a sample having a luminescence lifetime which is not as long as a pulsewidth of an excitation light.
2. Description of the Prior Art
Since, in the case where the luminescence lifetime of a sample is not as long as a pulsewidth of an excitation light, the luminescence shape is dependent upon the excitation light, and it is necessary to measure both a fluorescent shape I.sub.(t) and an excitation light shape P.sub.(t) and to obtain the true luminescence shape G.sub.(t) by the deconvolution operation to determine the luminescence lifetime. Here, the deconvolution operation is an operation in which G.sub.(t) is calulated by the following convolution integral equation (1): ##EQU1##
When I.sub.(t) and P.sub.(t) are measured at different times, the excitation light shape P.sub.(t) is different from that at the time when I.sub.(t) was measured, whereby an error is produced in the luminescence lifetime analysis. Accordingly, it is desirable to measure P.sub.(t) and I.sub.(t) at the same time.
Such an apparatus for measuring P.sub.(t) and I.sub.(t) at the same time is disclosed in Japanese Laid-Open Patent Application No. 59-72049/1984 (U.S. patent application Ser. No. 539,680) by the present applicant. This apparatus is provided with a time to amplitude converter (hereinafter referred to as a TAC) for use in the measurement of an excitation light shape and a TAC for use in the measurement of a transient shape, these two TACs being simultaneously operated by means of a start pulse; one TACs uses the photoelectron pulses due to the excitation light as stop pulses, and the other TAC uses the photoelectron pulses due to the emission from the sample as stop pulses; both TACs output voltages proportional to the time from the start of action to the receiving of the stop pulses, whereby the excitation light shape P.sub.(t) and the luminescence transient shape I.sub.(t) are measured at the same time on the basis of outputs from both TACs. However, since the length of a light passage for measuring P.sub.(t) is different from that of a light passage for measuring I.sub.(t), even though it is possible to carry out the measurement of P.sub.(t) and I.sub.(t) at the same time in the above described manner, it was found that the luminescence lifetime could not be accurately measured on account of the following reason:
The stop pulse of the TAC for use in the measurement of the excitation light shape is produced by an optical detector receiving a light from the light source while the stop pulse of the TAC for use in the measurement of the luminescence transient shape is produced by an optical detector receiving the emission from the sample. But, in general, the length of the light passage from the light source to the former optical detector is not equal to the length of the light passage from the sample to the latter optical detector. Now, provided that there is a difference in length of 10 cm between the light passages, this difference in length leads to a time difference of 0.3 nsec. This time difference leads to the shift of the difference between times when two stop pulses are added to a TAC. As a result, the time axis of the excitation light shape is shifted from the time axis of the fluorescence shape by 0.3 nsec, whereby the accurate measurement of the luminescence lifetime becomes impossible.
A method, in which a variable delay element is provided at an input side of one of the TACs and the delay time of this element is adjusted, can be thought of for the general measurement for eliminating such a shift between time axes. However, the variable delay element, which can be used for the place through which a high-speed pulse such as a stop pulse passes, has disadvantages in that it is extremely expensive, and its operating location being limited due to the passage of the high-speed pulse there-through, and is difficult to handle.